The present invention relates to a method of and an apparatus for detecting wheel spin. Such a method and apparatus may be used, for instance, in vehicle traction control systems.
In known traction control systems, for instance in vehicles driven by internal combustion engines, the drive to one or more of the driven wheels is reduced when wheel spin is detected so as to improve the drivability of the vehicle and prevent lateral instability. The speeds of the driven wheels are compared with a reference wheel speed and, when the speed of one or more driven wheel exceeds the reference speed by a detection threshold, the traction control system is actuated.
The reference speed may be determined in various ways. For instance, the reference speed may be determined as an average or mean value of the speeds of undriven wheels. Alternatively, the reference speed may be determined as the lower or lowest of the undriven wheel speeds.
Such systems perform well in many circumstances, particularly on surfaces of low coefficient of friction. However, a disadvantage can occur when cornering on a surface of high coefficient of friction. During cornering, the phenomenon of "weight transfer" occurs, whereby the weight on the outer wheels of the vehicle with respect to the centre of cornering increases whereas the weight on the inside wheels decreases. The weight on the inside driven wheel can be reduced to such an extent that wheel spin occurs prematurely. Indeed, the inside driven wheel can lift off the ground in extreme circumstances. The result of this is that unnecessarily early entry into traction control can occur when, in fact, adequate frictional contact between the outside driven wheel and the ground to prevent lateral instability is present.
The speed difference between the inside and outside wheels of a vehicle during cornering can be of the order of 2 or 3 kilometers per hour. In a typical traction control system, the spin threshold for actuating traction control can be between 4 and 6 kilometers per hour. By taking the reference wheel speed as the average of the undriven wheel speeds or the lower or lowest of the undriven wheel speeds, the detection threshold for traction control actuation can be reduced by a very substantial percentage, possibly as much as 50% or more, so that unnecessary traction control takes place on surfaces of relatively high friction.